The Cytogenetic Oncology Section has been examining specimens and tissue culture lines established from patients with hematologic malignancies and solid tumors in order to identify specific chromosomal changes associated with or diagnostic of these diseases. The breakpoints of these tumors indicate areas to look for new dominant oncogenes activated by translocations while the areas of deletions and loss of material by non-reciprocal translocations highlight areas to search for recessive oncogenes. These cytogenetic studies provide additional evidence that multiple genetic lesions are associated with the development of malignant tumors. We are presently conducting chromosomal in situ hybridization studies using either 3H labeled probes or biotinylated probes to localize viral integration sites, and to localize other genes that may be important to the development of malignant diseases. We are also using chromosome painting to study the relationship of various chromosomes involved in translocations (such as the 9 and 22 in CML) in interphase cells. We plan to study the potential role of DNA topoisomerases in med- iating illegitimate recombination in mammalian cells using the human c-abl protooncogene as a model system.